A digital storage oscilloscope (DSO) is one of the most commonly used instruments by engineers and scientists to display and analyze waveforms in electrical circuits. Often, a probe is utilized to connect the DSO to a circuit being measured and provides a mechanism for transmitting the signal in the circuit to the oscilloscope input.
There are certain important features or characteristics of probes worth noting. The first characteristic is the probe's loading. Since the probe is connected to a circuit under test, the loading can affect the circuit or change its operation. Since the probe usually shunts the circuit, the loading is best when the probe looks like a high impedance shunt element to the circuit. Another feature of interest is the ability of the probe to be connected at the correct measurement point and is usually addressed by making the tip of a probe as small as possible and by providing electrical contacts in the circuit that can be placed as close as possible to the desired measurement point. Still another feature is the impedance match of the probe output to the DSO channel input. When the probe's output impedance is matched to the DSO channel input, the transmitted signal passes into the scope undisturbed. Finally, and no less important is the general signal fidelity aspects internal to the probe such as magnitude and phase response.
Generally, when all of the aforementioned probe characteristics are good, the probe is useful for use with the DSO in the acquisition and measurement of signals occurring in the circuit under test.
As of late, high performance DSOs and probes are being utilized to measure extremely fast circuits and signals. The speed of these circuits is described by the frequency and risetimes of signals present in the circuits. When the frequencies of interest are very high and the risetimes are very short, it becomes very difficult to develop DSOs and probes with precision and accuracy sufficient for making accurate measurements. Specifically, many of the characteristics previously described become degraded.
At high frequencies, a probe's loading usually gets worse, causing the probe to interact more and more with the circuit being measured. When loading effects worsen, the probe sets up reflections due to impedance discontinuities in the circuit at the probing point. High speed measurements are often made on very small circuit features which make it difficult to access the desired measurement point. Often the desired measurement point is inside a chip and is completely inaccessible. It is not possible to make a perfect impedance match to the scope at high frequencies. This is a problem with the DSO design as well as the probe design. Finally, the probe exhibits more and more signal fidelity degradations internally as frequencies increase.
The problem of internal probe signal fidelity degradation has been addressed through the use of digital compensation. These are digital signal processing (DSP) methods that build filters to compensate for the probes response. The DSP methods are utilized inside the DSO using digital filters stored in the probe and are generated during a probe calibration step in the manufacture of the probe. Alternatively, these filters are derived from probe response information stored in the probe and generated during a probe calibration step in the manufacture of the probe.
In a coarse manner, the filters or responses that are stored in the probe have been utilized to compensate for probe loading effects. However, this compensation can only act in a coarse manner because the compensation methods currently used address only a fixed compensation that addresses the loading as if the probe was being utilized in the exact environment in which it was calibrated.
These filters may comprise analog or digital filters. Analog compensators, such as those described in U.S. Pat. No. 6,856,126 issued to McTigue, for example, comprise fixed filters, and are therefore not adjustable in accordance with the present invention. Even prior digital compensators fail to address the issues noted above. Typically, this type of digital probe compensation is handled in two ways—either the scope and probe combination is calibrated together on a given channel (the probe compensation is valid only so long as the probe is connected to the scope channel on which it is calibrated—it cannot be moved to another channel or another scope), or the scope/probe interface is designed to extremely high tolerances so that it can be ignored and is not considered and the probe compensator forms a fixed response.
US patents to Sekel (U.S. Pat. No. 6,870,359) and Pupalaikis (U.S. Pat. No. 6,701,335) provide for dynamic self calibration which is capable of compensating nearly from the probe tip through the scope (there will always be some portion of the tip not included in his calibration method). These methods are capable of accounting for the separate probe parts and the scope probe interface because the calibration signal could be injected near the tip and measured through the entire channel. However, this type of calibration requires complex circuitry and procedures to accomplish the compensation. While this method does provide a benefit of accounting for changing conditions of the parts over time and temperature as his does, the system is not one which can be easily implemented at, for example, a remote location not having an appropriate test and calibration fixture.
What is needed is a solution for addressing the probe loading problems in a general sense (i.e. considering the circuit under test). What is needed is a solution for probing inaccessible points. What is needed is a solution for dealing with the mismatch between the DSO and the probe. What is needed is a solution for determining the error bounds on measurements utilizing probes.
As a final note, the probe is generally manufactured with separate parts that are assembled. Often these parts are assembled by the scope and probe user at the time of use. Mechanisms have been provided for the storage of separate information corresponding to the individual probe parts. What is needed is a solution that combines measurements of separate probe portions into an aggregate probe measurement and using this probe measurement for effective probe compensation.